The invention relates generally to two stacked magnetoresistive (MR) sensors to provide a readback signal from the same data track in two-dimensional magnetic recording (TDMR) and a disk drive incorporating the sensors.
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a current-perpendicular-to-the-plane (CPP) sensor wherein the sense current is directed perpendicularly through the planes of the layers in the sensor stack. A CPP-MR sensor has a stack of layers that includes two ferromagnetic layers separated by a non-magnetic spacer layer. If the spacer layer is electrically conductive, for example copper (Cu) or silver (Ag), the sensor is referred to as a giant magnetoresistance (GMR) CPP sensor. If the spacer layer is an electrically insulating material, such as TiO2, MgO or alumina (Al2O3), the sensor is referred to as a magnetic tunnel junction sensor, also called a tunneling MR or TMR CPP sensor. In a CPP GMR or TMR sensor referred to as a single-free layer (SFL) sensor one ferromagnetic layer adjacent to the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference or pinned layer. The other ferromagnetic layer adjacent to the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the pinned-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance.
Another type of CPP GMR or TMR sensor is a dual-free layer (DFL) or “scissor” sensor that does not have a reference or pinned layer but instead has two free layers separated by a non-magnetic conductive or insulating spacer layer. With a sense current applied to a DFL sensor, the presence of an external magnetic field causes opposite rotation of the magnetizations of the two free layers relative to one another, sometimes referred to as a “scissoring” effect, and is detectable as a change in electrical resistance.
A proposed technology that uses multiple CPP-MR sensors is two-dimensional magnetic recording (TDMR). In one type of TDMR, two stacked sensors are located on a single structure and are aligned to read the same data track, with or without some slight intentional offset in the cross-track direction. The readback signals from the two sensors may be processed to double the signal for the same relative amount of electronic noise, or reduce sensor noise for the same relative amount of signal, and thus increase the signal-to-noise ratio (SNR). Each of the individual CPP-MR sensors in a TDMR read head structure is required to be located between two shields of magnetically permeable material that shield the sensors from recorded data bits that are neighboring the data bit being read. During readback, the shields ensure that each sensor reads only the information from its target bits. TDMR disk drives with two stacked CPP-MR sensors are described in U.S. Pat. No. 9,042,059 B1 and U.S. Pat. No. 9,230,578 B2, both assigned to the same assignee as this application.